Public Relations

White paper | Saving Energy using PICVs | June 2017
© Siemens Switzerland Ltd, 2017 3
Avoiding Overflow
Different Resistance Leads to Under- or Oversupply
In hydronic heating and cooling systems, the hot or cold
medium distributing the thermal energy from its generation
to the consumer (water, either plain or mixed with an agent
like glycol) is transported over piping sections of different
lengths and diameters. In the case of multi-story buildings,
the elevation to overcome may also vary. As a consequence,
the hydraulic resistance along the path from the energy
generator to each terminal unit is different.
To provide the required heating or cooling, each terminal
unit is designed for a certain flow. When the flow is too low,
the consumer does not receive enough energy
(undersupply). In the opposite case, when an overflow (or
oversupply) takes place, the flow is so high that the
terminal unit cannot sufficiently exchange the thermal
energy provided. As a consequence, the excess energy is
sent back to the energy generator, which is then unable to
operate at peak efficiency.
The Differences Are Normalized with Static Balancing
In order to ensure that every consumer receives the proper
amount of heating/cooling energy, hydraulic resistance is
introduced into the system. Conventionally, this so-called
balancing is done by installing manual balancing valves
(MBVs), which are installed in series to the standard
regulator valves. In this method, the hydraulic resistance of
the MBVs is dimensioned so that the system is perfectly
balanced for nominal operating condition. The system is
‘‘statically balanced.’’ However, this can only be achieved for
one given ‘‘ideal’’ operating condition (Fig. 2).
Overflow Still Happens in Spite of Static Balancing
The reality looks quite different, however. In statically
balanced systems, overflow may still occur in certain part-
load conditions.
For example, if some of the circuits are only half open (part-
load condition) and the rest are fully open (full-load
condition), an overflow takes place in the latter circuits,
which get excessive energy (Figure 3).
An overflow might last for quite some time before the room
temperature controller reacts to the increased or decreased
temperature. Such transient overflow phases usually occur
either due to a change of load (e.g., change of occupancy of
a room) or due to a change of setpoint (e.g., start-up phase
in the morning).
Overflow Leads to Energy Inefficiencies
Depending on the type of energy generators, this overflow
may lead to two negative side effects. First, overflow leads
to the transportation of water through the system that
doesn’t carry a proper amount of additional energy to the
consumers,
1
and hence a low temperature difference across
the heat exchanger. Second, in the case of chillers and heat
pumps, overflow causes inefficiencies in the energy
generators. Overflow of dedicated consumers can lead to a
return temperature lower than the nominal design value in
cooling mode and a return temperature higher than the
nominal design value in heating mode, decreasing the
energy efficiency of boilers and chillers by 2% and 3%,
respectively.
2
1
Heat transfer by the heat exchanger is directly
proportional to the flow rate and the temperature
difference across the heat exchanger. Flow rate and
temperature difference are inversely proportional to each
other in a closed system.
2
A decrease of the evaporation temperature of a chiller
below its design value by 1 degree decreases its
performance by around 3%. Increasing the condensing
temperature of a heat pump over its design value by 1
degree decreases its performance by around 2%.
Figure 3: When certain circuits are in part-load or closed,
others are in overflow (large blue arrows).
Figure 2: Statically balanced system
operating at the design operating point.